1. Field of the Invention
The present invention generally relates to a SAW filter circuit. More specifically, the present invention relates to a SAW filter circuit, and especially to a SAW filter circuit for transmission and reception, which can be used for small mobile communications apparatus such as a cellular phone.
2. Background Information
For example, conventional SAW filter circuits are shown in The Journal of the Institute of Electronics, Information and Communication Engineers, February 1993, and Japanese Laid-Open Patent Publications Nos. 10-93382, 2002-208835, and 2003-101384, which are hereby incorporated by reference.
The non-patent reference, The Journal of the Institute of Electronics, Information and Communication Engineers, February 1993, Vol. J76-A, No. 2, especially pages 245–251, discloses a SAW filter that is constituted from a ladder type circuit having one serial arm resonator and one parallel arm resonator. The serial arm resonator shows a double resonance characteristic. Specifically, frequency characteristics of Xp and Xs are shown. The frequency characteristic of XP is a deciding factor of the impedance Zp=jXp (j is an imaginary unit) of the parallel arm resonator. Further, the frequency characteristic of Xs, which is a deciding factor of the impedance Zs=jXs. This SAW filter circuit is designed so that the anti-resonance frequency fap of Xp and the resonance frequency frs of Xs may be in agreement, and it is constituted so that it may have a pass band in which a center frequency f equals fap which equals frs (i.e. f=fap=frs). Moreover, the SAW filter circuit is constituted so that a frequency of an attenuation pole of a low-pass side equals a frequency that makes Xp ‘0,’ and a frequency of an attenuation pole of a high-pass side equals a frequency that makes Xs into a local maximum. Thereby, near the center frequency, Xp is set to be larger than 50 ohms and Xs is set to ‘0.’ By having such a constitution, reduction of insertion loss in the pass band, reservation in the amount of attenuation in an attenuation band, and elimination of an alignment circuit are realized.
Japanese Laid-Open Patent Publication No. 10-93382 discloses a SAW filter circuit that is constituted from a ladder type circuit having a plurality of parallel arm resonators. In this SAW filter circuit, ground sides of the parallel arm resonators are made to have equal potential on a common bonding pad. Further, the bonding pad is electrically connected to the ground of a package by a common bonding wire (inductance LE). Thereby, when the standard amount of attenuation in a low-pass side attenuation band is eased, it is able to realize a redress in the amount of attenuation in a high-pass side attenuation band and improvement of insertion loss in a pass band.
Japanese Laid-Open Patent Publication No. 2002-208835 discloses a SAW filter circuit that is made of a band-pass ladder type SAW filter and a two-terminal pair circuit. The two-terminal pair circuit is composed of three inductors, and the band-pass ladder type SAW filter and the two-terminal pair circuit are serialized. In this SAW filter circuit, by having these three inductors in the two-terminal pair circuit, two attenuation bands can be formed for each of a high-pass side and a low-pass side of the pass band. The amount of attenuation can be improved not only in the attenuation band of the high-pass side but also in the attenuation band of the low-pass side.
Japanese Laid-Open Patent Publication No. 2003-101384 discloses a SAW filter circuit that is constituted from a ladder type circuit having a plurality of serial arm resonators and a plurality of parallel arm resonators. In this SAW filter circuit, a ground side of each parallel arm resonators is electrically connected to a bonding pad through an individual inductor L (bonding wire). The bonding pad is electrically connected to the ground of a package through an inductor Lp (bonding wire), thereby the ground side of each parallel arm resonators is electrically connected to the ground of the package. Moreover, in this SAW filter circuit, the inductance value needed for the bonding wire is reduced by arranging a capacitor between the ground side of the parallel arm resonator located on an input stage and the ground side of the parallel arm resonator located on an output stage.
Generally, in a mobile telecommunications terminal, a transmitting band and a receiving band have to be set to be wide, and the interval between the transmitting band and the receiving band has to be set to be narrow. Therefore, it is necessary to secure a sufficient amount of attenuation in a band to which the transmitting band and the receiving band approach. In other words, when the transmitting band is on a low-pass side and the receiving band is on a high-pass side, it is necessary to constitute each SAW filter so that sufficient attenuation characteristics may be acquired at the edge of the high side or the low side of each pass band, while securing the amount of attenuation by the high band of a transmission filter or the low band of a reception filter.
With the ladder type circuit composition given in the non-patent reference, The Journal of the Institute of Electronics, since the transmission filter and the reception filter each has only a single attenuation pole on each of the low side attenuation band and the high side attenuation band, it is realized that it is difficult to acquire the sufficient attenuation band width for each attenuation band.
With the circuit compositions given in the first two Japanese Laid-Open Patent Publications, 10-93382 and 2002-208835, the attenuation band width of the attenuation band satisfying the required amount of attenuation tends to become wider as the inductance value becomes larger. This inductance is made of a bonding wire or a strip line. When this inductance is constituted by the bonding wire, the inductance value becomes larger as the bonding wire becomes longer, and the attenuation characteristics become highly dependent on the length of the bonding wire.
With the circuit composition given in Japanese Laid-Open Patent Publication 2003-101384, by arranging the capacitor between the ground side of the parallel arm resonator located on the input stage and the ground side of the parallel arm resonator located on the output stage, reduction in the inductance value needed for the bonding wire and reduction in the length of the bonding wire are attempted. However, in order to reduce the length of the wire bonding, it is necessary to use a considerably large capacitor to obtain a large capacitance value, and therefore, downsizing of a SAW filter equipment is difficult. In this respect, a SAW filter circuit that is suitable for miniaturization of the SAW filter equipment, and has good attenuation characteristics in the attenuation band is required.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved SAW filter circuit. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.